Power amplifiers include non-linear components. For example, an equivalent resistance at a node in the signal path of a power amplifier varies as the power of the signal varies. Similarly the equivalent capacitance at a given node varies nonlinearly with power level. Thus, signals amplified by the power amplifier include phase distortion and amplitude distortion.
A classic feedback loop for correcting phase distortion in a power amplifier is complex, expensive, consumes much power, and occupies much space. This classic feedback loop measures the phase of an amplified signal, compares the measured phase to a reference phase to determine a phase error, generates a phase error signal, and modifies an input signal (or modifies the power amplifier) to correct the phase error and generate an undistorted amplified signal.
Thus, in conventional power amplifiers there is no way to correct phase distortions without first measuring the phase distortion of the amplified signal.